Ion implantation has become the technology preferred by industry to dope semiconductors with impurities in the large scale manufacture of integrated circuits. A typical ion implanter comprises three sections or subsystems: (i) a terminal including an ion source region which produces the ion beam of desired current and energy, (ii) a target chamber which contains the semiconductor wafer to be implanted by the ion beam, and (iii) a beamline assembly, located between the terminal and the target chamber, which conditions the ion beam output by the ion source and directs the conditioned beam toward the target wafer.
The entire ion implantation process takes place within a vacuum to insure consistent implants and prevent particle contamination. High vacuum pumps are provided for this purpose. Typically, individual high vacuum pumps are provided for the source region, the beamline assembly, and the target chamber. At the region of the target chamber, load locks are used to insert wafers into and withdraw wafers from the target chamber to avoid repeated pressurization and depressurization of the target chamber.
The vacuum systems which are used to evacuate the respective regions of the ion implanter to which they are connected must exhaust the air which is removed from these regions. The exhaust is typically vented to the environment outside of the ion implanter enclosure to accommodate both safety and contamination concerns.
The terminal is necessarily operated at a high voltage to facilitate ion extraction from the source and ion acceleration toward the target chamber. Typically, this voltage is in the range of 80,000 to 100,000 volts (80-100 kilovolts (kV)). The outer enclosure of the ion implanter, however, is electrically grounded for safety reasons. Accordingly, air which has been evacuated from the source region by the source vacuum pump must be vented from the high voltage terminal, through the grounded enclosure, and out to the external environment.
Typically, an exhaust tube connects the source vacuum pump to vent air evacuated from the source region. The exhaust tube extends from the source vacuum pump, through the high voltage terminal housing, and through the grounded outer enclosure to the external environment. The portion of the exhaust tube which extends between the high voltage terminal housing and the grounded outer enclosure must be constructed of a non-conductive, insulating material to prevent electrical contact between the high voltage terminal housing and the grounded outer enclosure.
Over the extended course of operation of the ion implanter, the interior walls of the source vacuum exhaust tube become contaminated with residuals of the source process and other airborne material which has been evacuated during evacuation of the source region. Accumulation of these contaminants, especially hydrocarbons, may eventually compromise the insulative quality of the non-conductive tube, which could result in leakage currents from the terminal to ground, or, worse, high power electrical discharges resulting from arcing between the terminal and the enclosure. To prevent such occurrences, the tube must be either cleaned or replaced to restore system integrity. Because cleaning the tube is time consuming and increases the risk of exposure to the contaminants, the tube is typically replaced.
Accordingly, it is an object of the invention to provide an improved exhaust tube for use in an ion implanter, which is easily maintained to preserve its electrically insulative qualities.
It is a further object of the present invention to provide an exhaust tube assembly for use in an ion implanter which is made economically more practicable by a construction which provides a permanent, reusable outer tube and an inexpensive replaceable and disposable inner tube. The inner tube provides means to contain contaminants and is of a corrugated configuration so as to provide an elongated path for electrical leakage, or flashover, thereby extending the time between maintenance (replacement) intervals as compared to an uncorrugated configuration.
It is still a further object of the present invention to provide an exhaust tube assembly having a dual walled construction which facilitates maintenance of the integrity of its electrical non-conductivity, and which minimizes contaminant collection therein.
It is still a further object of the present invention to provide a dual walled exhaust tube assembly, wherein a corrugated inner contamination containment tube is relatively thin-walled as compared to a more robust permanent outer tube which provides effective containment in the event of a puncture of the inner tube.